Some relatively responsible people at MIT did a sodium drop for years by walking into the middle of Longfellow bridge and then dropping the sodium into the Charles, where the metal lump will burst apart but no fragments are unlikely to reach shore. Couple of really dumb MIT students from the TEP independent living group got in very big trouble a couple of years back after they did their own "sodium drop" except off a pier. A chunk of sodium ended up on the shore, gathered water to look kind of like a metal slug (since it's hygroscopic) and then got picked up by a river cleanup group, who put it in a trash can on their boat. The can caught fire and TEP got sued, and the Boston Press of course had a field day claiming the sodium was "stolen" (in all cases it was purchased) and that the original MIT sodium drop was responsible because that's the top result on teh google.

Yeah thats about the right size boom.Had to do this on a parking lot when my mom took over teaching chemistry at a private school aeons ago now. There was a lot of old and dangerous stuff in the store room. The hazmat folk told her it would cost way too much for their trouble and if there was an open field just to react the stuff off as it was still under their size limit. So got to see pretty colored flames from most of the stuff as it was small thumb sized hunks. Dump it out, toss water on it and go oooh cool.The sodium on the other hand was a 1 pound hunk and we had an old 60 gallon steel drum that had been in use a garbage can for years. That got filled with water, then me still being young and invincible dumped in the sodium and took off at good clip. I looked back right after the boom to see the drum some 7ish feet up in the air and water spraying out the bottom. It put a good 6 inch tear in the steel. It was pretty freaking cool, fun, and scary.

Back in high school somebody carved a good sized hunk of Na off of the giant hunk that was in the storage room. They put in on the sidewalk and inverted a small garbage can that was packed tight with snow over it. I guess that it shot up quite a ways, but there were only a few witnesses...

When I was in high school, there were some dangerous things in the chemistry lab stockroom, but sodium wasn't one of them. Not because the school was afraid to have them (especially with Happy Jack Morgan as the chem-phys teacher :-)), but I think there was a concern that somebody might carve off pieces of it for activities like that, just as somebody walked off with a jar of iodine crystals, though that perpetrator* had enough sense not to bring the resulting nitrogen tri-iodide back to school instead of blowing it up with his friends.

*(No, I really wasn't the perpetrator, and I forget which of my friends might have been, and I didn't get to be around when they used it. My unindicted co-conspirator and I mainly did smoke bombs and fireworks of different colors.)

We had little Dougie Stanton for chem and he would have us make nitrogen tri iodide in class and encourage us to paint it on objects around the school -- this ended after somebody coated the principal's door knob...The great Na heist was under the nose of the nicest, smartest, but most gullible fellow in the school.

I know someone who managed to make it "rain fire" in a lab at one point. They ended up with a smallish sodium boom that still sprayed bits of it onto the ceiling. They got those down the simple way: squirt bottle.

We used to shoot a pound-sized cylinder of sodium into a sizable river out of a mortar made from a carpet tube (the thing carpet is rolled around, heavy cardboard) charged with black powder.

It's fun but mildly guilt-inducing until rationalized as follows: a pound of sodium is about 20 moles. Dissolved in a cubic meter of water, it makes a 0.02 molar solution, which isn't very strong. There's far more water than that involved, and the water is somewhat buffered with whatever's dissolved in it, so the change in pH, even fairly close to the reaction, would be very small. Of course, the reaction is rapid and violent, thus not good for anything that was close by, but a river is a big place.

Edited to add: I think this would be a very bad idea in water without a current, because concentrated parts of the NaOH plume would be dense enough to sink to the bottom, making it very unpleasant there until it dissipated.

I'm no chemist but the first thing I thought when I saw this was what my science teacher said after he showed us a small sliver with water. Someone inevitably commented that it would be awesome to throw a big chunk in a creek and he told us that some clever clogs had done just that and poisoned the stream.